The present invention relates to a Dewar vessel (double structural container, which include a vacuum space, for using very low substances) for containing SQUId (Superconducting Quantum Interference Devices) which is used for a transducer of magnetic signal-transducer, a material property-measurement instrument for measuring the magnetic permeability of a substance, magnetic field generated from a human body or a living being.
Here, the SQUId is held in a very low temperature by using liquid helium or liquid nitrogen in an adiabatic cooling container (cryostat), and a loop of the SQUId including Josephson junctions is driven by applying DC current to the SQUId as a bias current. If magnetic fluid from the outside is input via a pick-up coil or an input coil, an orbital current is induced, and the very weak magnetic fluid applied from the outside is largely amplified by the quantal interference effect of the Josephson junctions. The element is used a magnetic field transducer for measuring minimal magnetic fluid.
Japanese Patent Application Laid-Open Hei 6-21520 discloses the composition of a radiation shield.
Although a conventional SQUId magnetic field measurement Dewar vessel is made of non-metallic material in order to reduce magnetic noises, it is necessary to use metal such as copper because a radiation shield member must has high heat conduction. However, if the shield member is metallic, changes in very weak magnetic field causes a weak induced current in the radiation shield, which in turn causes a measurement error. This has been a large problem in the magnetic field measurement. To solve this problem, in Japanese Patent Application Laid-Open Hei 6-21520, thin stripes insulated each other are used in the radiation shield.
Further, since the radiation shield is made of very thin foil in order to cause induced current, a substrate to which the metal foils are fixed is thin so as to be easily processed in fabrication of the radiation shield. Accordingly, the stiffness of the substrate of the shield is also weak. Thus, the radiation shield easily contacts an inner container, which in turn increases the evaporation amount of liquid helium which has been poured into the inner contain, and the consumption amount of liquid helium
Furthermore, if thin stripes insulated each other, which are fixed to a substrate, used as the radiation shield, the width of each stripe the SQUId is reduced near the SQUID in order to decrease an induced current due to changes in magnetic field, and this increase the resistance of conducting heat in each metallic stripe, and the temperature of the radiation shield. In addition, the groove portions in which the meatic stripes do not esist are not sufficiently cooled, and the above described problems will reduce the stiffens of the radiation shield this may increaseas the probability of that the radiation shield contacts the very low temperatureh internal container.
The present invention has been achieved in consideration with the above problems, is aimed at providing a Dewar vessel which is easily fabricated, and consists of a high performance radiation shield of small induced current, and at preventing increase of necessary liquid helium.
Thus, the present invention provides an minimal magnetic-field measurement Dewar vessel including an inner container which contains a superconducting device and cooling medium; a radiation shield which is composed of a plurality of thin line stripes electrically insulated each other and in a high stripe dense state; the inner container, an outer container which surrounds the radiation, and in which a vacuum state is kept, and a cooling apparatus which cools the radiation shield;
wherein the number density and the width of, each thin line stripe arranged surrounding the superconducting device is larger and smaller than those values of a thin line stripe arranged in other areas, respectively. Further, an electrical insulation supporting means is provided so as to keep a distance between and the radiation shield and the inner container at a predetermined value.
In accordance with the present invention, a radiation shield is composed so that a plurality of a thin stripe members of high thermal conductivity is fixed to an electrical insulation substrate, and gaps between the stripe members gradually narrower as the radiation shield are nearer to a superconducting apparatus. Accordingly, in the longitudinal direction, since the width of the stripe members at the portion of a cooling means and a thermal contact is large, if the end part of the substrate in the longitudidal direction is cooled, the whole system can be well colled even in the other end part. Moreover, since the width of each stripe member becomes very thin, and the tripe members electrically insulate each otter, even if an induced current is caused due to changes in magnetic field as the SQUID, the induced current is very small due to the thin width of the stripe members. Thus, it has becomes possible to provide a thermal shield plate causes a very small magnetic measurement shield.
Further, the stripe members of high thermal conductivity are fixed while electrically insulating each other, and are arranged so that each groove line is not located at the same position on both surfaces of the substrate. Therefore, there is always a stripe member directly is formed directly above each groove via the substrate. Accordingly, each groove on the surface of the substrate is always cooled by a corresponding stripe member on the surface of the substrate, the temperature increase due to insufficient cooling of each groove lien can be prevented.
Furthermore, by support the radiation shield by an electrical insulation support means of high stiffness in order to keep the distance between the internal container and the radiation shield at a predetermined, it can be solved that increase of helium evaporation when the radiation shield of low stiffness thermally contacts the internal container during operations or a transportation of the Dewar vessel, and the heat transfers into the internal container from the radiation shield.